Copolyester, and hollow container and stretched film made thereof

ABSTRACT

A copolyester which comprises, as main components, terephthalic acid as a dicarboxylic acid component and ethylene glycol as a diol component and which is characterized by: 
     (1) isophthalic acid as a dicarboxylic acid component being from 0.5 to 3.0 mol %, 
     (2) diethylene glycol as a diol component being from 1.0 to 2.5 mol %, 
     (3) the intrinsic viscosity being from 0.60 to 1.50 dl/g, 
     (4) the concentration of terminal carboxyl groups being at most 18 eq/ton, and 
     (5) the content of a cyclic trimer being at most 0.40% by weight.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a copolyester useful for bottles, filmsor sheets. More particularly, the present invention relates to acopolyester which is capable of presenting a molded product with goodproductivity in its molding and having excellent heat resistance andmechanical properties and which shows excellent productivity in itsproduction, and molded products made thereof.

2. Discussion of Background

Polyethylene terephthalate (hereinafter referred to simply as "PET") isexcellent in the mechanical strength, chemical stability, transparencyand sanitation and is light in weight and inexpensive. Therefore, it iswidely used as packaging material in the form of various sheets andcontainers. Its use as containers for soft drinks, fruit juices, liquidseasonings, edible oils, liquors and wines has been particularlyremarkable in recent years.

For example, in a case of a bottle, such PET is molded into a preformfor a hollow molded product by an injection molding machine, and thispreform is then subjected to stretch blow molding in a mold having apredetermined shape. In a case where the content of the bottle requireshot filling as in the case of a fruit juice beverage, it is common toconduct heat setting in the blow mold or in a separate mold to form aheat set bottle.

However, conventional PET chips used for molding usually containoligomers in an amount of from 1 to 2% by weight in the case of meltpolymerization chips and from 0.5 to 1.0% by weight even in the case ofsolid-state polymerization chips, as the amount of a cyclic trimer asthe main component of the oligomers. Such oligomers tend to deposit onand contaminate the apparatus such as the mold during the moldingoperation. Contamination of the mold, etc. is likely to cause blushingor surface roughening of the resulting molded product. Therefore, it isnecessary to frequently clean the mold, etc.

Heretofore, it has been attempted to reduce oligomers by prolonging thesolid-state polymerization time or by increasing the amount of thecatalyst. However, reduction of oligomers by such a method is ratherlimited and is not economical.

On the other hand, many copolyesters having properties similar to PET,such as a copolyester using terephthalic acid and isophthalic acid asthe dicarboxylic acid components and a copolyester using ethylene glycoland diethylene glycol as the diol components, are also known. However,no copolyester has been specifically known with the oligomer contentbeing reduced to a certain extent and having physical properties equalor superior to PET. Further, it has not been known that bycopolymerizing a small amount of isophthalic acid to PET, the oligomercontent of the copolyester obtainable by melt polymerization, followedby solid-state polymerization, can be lowered as compared withhomopolymerized PET.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a copolyester whichhas a low oligomer content and produces no substantial oligomers duringthe molding operation and thus scarcely contaminates the mold, etc.during the molding operation and which has heat resistance at leastequal to conventional PET and has a high polymerization rate and anoligomer-reducing rate during solid-phase polymerization, and thuspresents high productivity.

The present inventors have conducted extensive studies to accomplish theabove object and as a result, have found a copolyester with certainspecific physical property ranges having small amounts of isophthalicacid units and diethylene glycol units incorporated to conventional PET.The present invention has been accomplished on the basis of thisdiscovery.

That is, the present invention provides the following copolyester andmolded products made thereof.

A copolyester which comprises, as main components, terephthalic acid asa dicarboxylic acid component and ethylene glycol as a diol componentand which is characterized by:

(1) isophthalic acid as a dicarboxylic acid component being from 0.5 to3.0 mol %,

(2) diethylene glycol as a diol component being from 1.0 to 2.5 mol %,

(3) the intrinsic viscosity being from 0.60 to 1.50 dl/g,

(4) the concentration of terminal carboxyl groups being at most 18eq/ton, and

(5) the content of a cyclic trimer being at most 0.40% by weight.

The above copolyester is preferably produced by solid-statepolymerization of the following prepolymer.

A prepolymer which comprises, as main components, terephthalic acid as adicarboxylic acid component and ethylene glycol as a diol component andwhich is characterized by

(1) isophthalic acid as a dicarboxylic acid component being from 0.5 to3.0 mol %,

(2) diethylene glycol as a diol component being from 1.0 to 2.5 mol %,

(6) the intrinsic viscosity being from 0.50 to 0.70 dl/g, and

(7) the concentration of terminal carboxyl groups being from 15 to 30eq/ton.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, the present invention will be described in detail with reference tothe preferred embodiments.

With respect to terephthalic acid and ethylene glycol as the maincomponents for the copolyester of the present invention, conventionalmaterials commonly used for PET, may be employed.

Materials for isophthalic acid units include isophthalic acid, esterssuch as dimethyl isophthalate and diethyl isophthalate, isophthalicacids having the nuclei substituted by alkyl, alkoxy, aryl, aralkyl,halogen, etc., such as 5-t-butylisophthalic acid and 5-methylisophthalicacid, 5-sulfonylisophthalic acid and its sodium salt. Among them,isophthalic acid or dimethyl isophthalate is particularly preferred.

On the other hand, diethylene glycol (hereinafter referred to simply as"DEG") is produced as a byproduct from ethylene glycol during thepolymerization reaction. Therefore, the content of the DEG component canbe controlled simply by properly selecting the reaction conditions,additives, etc., other than a case where DEG or its ester-formingderivative is used in a prescribed amount as the polymerizationmaterial. Especially in the case of the copolyester of the presentinvention, the melt polymerization temperature can be lowered by theeffects obtained by the addition of isophthalic acid (hereinafterreferred to simply as "IPA"), whereby the amount of DEG to be formed asa byproduct, can be easily controlled to a low level. Further, formationof DEG can be controlled by adding a small amount of an additive, forexample, a tertiary amine such as triethylamine, tri-n-butylamine orbenzyldimethylamine, a quaternary ammonium hydroxide such astetraethylammonium hydroxide, tetrabutylammonium hydroxide ortrimethylbenzylammonium hydroxide, or a basic compound such as lithiumcarbonate, sodium carbonate, potassium carbonate or sodium acetate. Onthe other hand, formation of DEG can be promoted and the DEG content canbe increased by adding a small amount of an inorganic acid such assulfuric acid to the polymerization material. Such an additive tocontrol the amount of formation of DEG is used usually within a range offrom 0.001 to 10% by weight, preferably from 0.005 to 1% by weight,based on the entire polymerization material, as the case requires.

The copolyester of the present invention has a feature that theproportion of IPA in the total dicarboxylic acid component is from 0.5to 3.0 mol %, preferably from 1.0 to 2.5 mol %, and the proportion ofDEG in the total diol component is from 1.0 to 2.5 mol %, preferablyfrom 1.2 to 2.3 mol %. If the respective proportions are less than theabove ranges, the oligomer-reducing rate in the solid-statepolymerization step for the production of the copolyester of the presentinvention, is low, and the effects to reduce the oligomer deposited onthe mold during the molding operation tend to be inadequate, and nosuperiority over conventional PET tends to be observed. On the otherhand, if the proportions exceed the above ranges, the heat resistance ofa molded product tends to be poor, and when a stretched molded productis subjected to heat setting, improvement in the heat resistance due tothe heat setting effects tends to be little, such being undesirable.Especially when the amount of IPA exceeds 10 mol %, the decrease in theglass transition temperature of the copolyester itself tends to besubstantial, the heat resistance tends to be substantially low, andformation of oligomers derived from IPA tends to be remarkable, suchbeing undesirable.

The intrinsic viscosity of the copolyester of the present invention isusually within a range of from 0.60 to 1.50 dl/g, preferably from 0.65to 1.20 dl/g, more preferably from 0.70 to 1.00 dl/g, as measured at 30°C. in a solvent mixture of phenol/tetrachloroethane (1/1 by weightratio). If the intrinsic viscosity is less than 0.60 dl/g, a moldedproduct obtained from such a copolyester will have no adequate practicalstrength. On the other hand, if the intrinsic viscosity exceeds 1.50dl/g, the melt viscosity tends to be too high, and the shear heatgeneration during injection or extrusion molding or in a moldingmachine, tends to be substantial, whereby oligomers which have once beenreduced, will again be produced in a substantial amount, andconsequently, no improvement for the prevention of contamination of themold, etc. will be obtained, such being undesirable.

The content of oligomers in the copolyester of the present invention isusually at most 0.40% by weight, preferably at most 0.35% by weight,more preferably at most 0.32% by weight, most preferably at most 0.30%by weight, in terms of the content of a cyclic trimer as the maincomponent of the oligomers. When the copolyester with a cyclic trimercontent of at most 0.40% by weight is subjected to molding, animprovement in the prevention of contamination of the mold, etc. will beobserved. In general, the smaller the content of the cyclic trimer, thebetter the improvement in the prevention of contamination of the mold,etc. For example, even when the content of the cyclic trimer exceeds0.35% by weight, a substantial improvement in the prevention ofcontamination can be observed, although the improvement may not besufficient if the content exceeds 0.40% by weight, and if the content ofthe cyclic trimer exceeds 0.50% by weight, contamination tends to beremarkable. Therefore, with respect to the improvement for theprevention of contamination of the mold, etc., the content of the cyclictrimer being at most 0.40% by weight or not, will be an index.

The degree of contamination of the mold is closely related to thecontent of the cyclic trimer in the molded product. Namely, if thecontent of the cyclic trimer in the molded product is not more than0.40% by weight, a substantially sufficient improvement for theprevention of contamination of the mold, can be obtained, and if it isat most 0.35% by weight, the improvement for the prevention ofcontamination of the mold is remarkable. At a level of 0.45% by weight,a substantial improvement for the prevention of contamination can beobtained although the improvement may not be adequate. At a level of0.55% by weight or higher, contamination will be remarkable. Withrespect to the improvement for the prevention of contamination of themold, the content of the cyclic trimer in the molded product being atmost 0.45% by weight or not, will be another index.

The density of the copolyester of the present invention, is usually atleast 1.37 g/cm³, preferably at least 1.38 g/cm³, more preferably atleast 1.39 g/cm³, as measured at 25° C. by means of a density gradienttube employing a solvent mixture of carbon tetrachloride/n-heptane. Ifthe density is less than 1.37 g/cm³, the non-crystalline fraction in thecopolyester tends to be high, whereby the solid-state polymerization orheat treatment tends to be inadequate, and oligomers can not adequatelybe reduced.

With the copolyester of the present invention, the concentration ofterminal carboxyl groups (hereinafter referred to simply as "AV") isusually at most 18 eq/ton, preferably at most 16 eq/ton, more preferablyat most 14 eq/ton. A copolyester having the oligomer content efficientlyreduced during the solid-state polymerization, has AV within this range,when AV is within this range, the effects to reduce oligomers depositedto the mold, etc. during the molding of the copolyester of the presentinvention, will be substantial, and the moisture resistance, thermalstability, etc., will also be improved. If AV exceeds the above range,the effects to reduce oligomers deposited on the mold, etc. during themolding, tend to be low, and the moisture resistance, thermal stability,etc., will also be low, such being undesirable. In the preparation ofthe copolyester of the present invention, a copolyester subjected tosolid-state polymerization will be referred to as a "prepolymer". AV ofthe copolyester of the present invention can be controlled by themoisture-controlling conditions, the crystallization conditions and thesolid-state polymerization conditions for the prepolymer and AV of theprepolymer. For example, AV can be controlled by changing thetemperature, time or pressure of each of steps for the moisture control,crystallization and solid-state polymerization of the prepolymer. Asother specific examples, AV of the copolyester of the present inventioncan be lowered, for example, by lowering AV of the prepolymer, bysuppressing hydrolysis during the crystallization step or thesolid-state polymerization step by reducing the amount of waterimpregnated during the moisture control of the prepolymer, or byincreasing the ethylene glycol concentration in the inert gas streamused for the solid-state polymerization. On the other hand, AV of thecopolyester of the present invention can be increased, for example, byincreasing AV of the prepolymer, by increasing the amount of waterimpregnated during the moisture control of the prepolymer, byaccelerating the hydrolysis during the crystallization step or thesolid-state polymerization step e.g. by conducting the crystallizationwhile introducing steam, or by reducing the ethylene glycolconcentration in the inert gas stream used for the solid-statepolymerization.

In the copolyester of the present invention, the proportion of theterminal carboxyl groups to the total terminal groups (hereinafterreferred to simply as AV/TEV) is preferably within a range of from 7 to25 equivalent %, more preferably from 8 to 20 equivalent %, mostpreferably from 10 to 18 equivalent %. When AV/TEV is within the aboverange, the solid-state polymerization rate or the oligomer-reducing rateis high during the production, whereby the productivity will beimproved, and the amount of the oligomers to be produced as byproductsduring the molding can thereby be further reduced. This AV/TEV ratio canbe controlled in the same manner as for the above described AV.

In the production of PET by solid-state polymerization, it is common toset the solid-state polymerization conditions so that the solid-statepolymerization rate (the degree of increase of the intrinsic viscosityper unit time) is as high as possible to attain the desired intrinsicviscosity in a short period of time in order to increase theproductivity. It has been well known that a close relation existsbetween the solid-state polymerization rate and AV and AV/TEV relatingto PET. In general, if AV is the same, the solid-state polymerizationrate becomes highest when AV/TEV is within a range of from 30 to 35equivalent %. The optimum AV to bring the solid-state polymerizationrate to the highest level, may automatically be determined by theintrinsic viscosity and the optimum value of AV/TEV.

Accordingly, in general, it is preferred to conduct solid-statepolymerization under such a condition that AV and AV/TEV would be theoptimum values to bring the solid-phase polymerization rate to thehighest level. Therefore, AV is usually at a level of from 20 to 35eq/ton, and AV/TEV is usually at a level within a range of from 25 to 40equivalent %.

Whereas, with respect to the oligomer-reducing rate in solid-statepolymerization, it has been found as a result of an extensive study bythe present inventors that the lower the AV and the lower the AV/TEV, inother words, the higher the concentration of terminal hydroxyl groups,the higher the oligomer-reducing rate. Accordingly, when the copolyesterof the present invention is prepared by solid-state polymerization, itis advisable that AV and AV/TEV of the prepolymer to be used, are as lowas possible with a view to reducing the oligomers. However, as mentionedabove, with respect to the solid-state polymerization rate, there existsthe optimum values for AV and AV/TEV. Therefore, it is necessary to setAV and AV/TEV so that both the solid-state polymerization rate and theoligomer-reducing rate will be high. Due to the effects obtained by thecopolymerization of a small amount of IPA, the solid-statepolymerization rate is basically high as compared withhomopolymerization of PET. Therefore, unless AV and AV/TEV are set atextremely low levels, even if AV and AV/TEV are set at low levels toslow down the solid-state polymerization rate to some extent, thesolid-state polymerization rate would be substantially the same level asin the case of usual homopolymerization of PET. The copolyester of thepresent invention obtained by solid-state polymerization already has thedesired intrinsic viscosity, and therefore it is unnecessary to subjectit solid-state polymerization again. Therefore, AV and AV/TEV may bevery low, which is advantageous, since the resistance against hydrolysisis thereby improved.

When the copolyester of the present invention contains germanium atoms,the content of germanium atoms is usually preferably from 30 to 60 ppmby weight, more preferably from 35 to 55 ppm by weight, most preferablyfrom 40 to 50 ppm by weight. On the other hand, when antimony atoms arecontained, the content thereof is usually preferably from 150 to 300 ppmby weight, more preferably from 170 to 280 ppm by weight, mostpreferably from 200 to 250 ppm by weight. When compared with homopolymerof PET, the copolyester of the present invention is superior in theproductivity since the solid-state polymerization rate is high and thesolid-state polymerization time required for reducing the oligomercontent to the same level in the production, may be short when the sameamount of germanium atoms or antimony atoms is contained. Besides, theamount of oligomers produced as byproducts during the molding operationis small, and the effects to improve the prevention of contamination ofthe mold are improved. However, the larger the content of germaniumatoms or antimony atoms, the higher the oligomer-reducing rate, thehigher the solid-state polymerization rate and the more the improvementin the productivity in the production of the copolyester of the presentinvention. However, the amount of oligomers produced as byproductsduring the molding operation, tends to increase. To the contrary, thesmaller the content of the germanium atoms or antimony atoms, thesmaller the amount of oligomers produced as byproducts during themolding operation, but the productivity tends to decrease during theproduction of the copolyester of the present invention. When thecontents of germanium atoms and antimony atoms are within the aboveranges, both the productivity during the production of the copolyesterof the present invention and the reduction of the amount of oligomersproduced as byproducts during the molding operation, will besubstantially improved, such being desirable. Such germanium atoms orantimony atoms, are the ones derived from a germanium compound or anantimony compound used as a polymerization catalyst for the copolyesterof the present invention, which will be described hereinafter, and whichis taken into the polymer.

With respect to the thermal properties of the copolyester of the presentinvention, the glass transition temperature (Tg) is desired to beusually within a range of from 72° to 82° C., preferably from 74° to 82°C., more preferably from 75° to 82° C., and the low temperaturecrystallization peak (Tc) is desired to be usually within the range offrom 130° to 210° C., preferably from 150° to 205° C., more preferablyfrom 160° to 200° C. For the determination of such Tg and Tc, 5 mg of asample of the copolyester is heated from room temperature to 285° C. ata rate of 20° C./min by a differential scanning calorimeter (hereinafterreferred to simply as "DSC") and then melted and maintained at 285° C.for 3 minutes. Then, the sample is quickly taken out and immediatelyimmersed into liquid nitrogen, maintained therein for one minute andthen left to stand at room temperature for 30 minutes to one hour. Thesample returned to room temperature, is returned to the temperature andagain heated from room temperature at a rate of 20° C./min, whereupon Tgand Tc are determined from the specific heat modification behavior dueto the glass transition and from the heat generation behavior due tocrystallization, from the calorie curve thereby measured. Morespecifically, Tg is the temperature at the intersection of the tangentline at an intermediate point of the specific heat modification due toglass transition with the tangent line at a point prior to the specificheat modification, and Tc is the temperature at which the heatgeneration per unit time becomes the maximum at the heat generation peakdue to crystallization. When Tg and Tc are within the above ranges, itis readily possible to obtain a proper level of crystallinity as normalmolding material and to bring the heat resistance of a molded productobtainable by heat setting after stretching to a level of at least equalto homopolymerized PET.

With respect to the thermal properties of a molded product obtained byinjection molding or extrusion molding of the copolyester of the presentinvention, it is desired that Tg is usually within a range of from 72 to82° C., preferably from 74° to 82° C., more preferably from 75° to 82°C., and Tc is usually within a range of from 130° to 180° C., preferablyfrom 130° to 170° C., more preferably from 135° to 165° C., as measuredin the same manner as described above using DSC. When Tg and Tc arewithin these ranges, the molded product has a proper crystallinity as aordinary molded product, and it is easy to bring the heat resistance ofa molded product obtained by heat setting after stretching to a level ofat least equal to homogenized PET. With respect to the copolyester andits molded product, there is a tendency that Tc is lower with the moldedproduct. This phenomenon is considered to indicate that the shearhistory during the molding remains in the molded product and does notcompletely disappear by the melting operation at a level of 3 minutes at285° C. by DSC.

The content of acetaldehyde in the copolyester of the present inventionis desired to be usually at most 7 ppm by weight, preferably at most 5ppm by weight, more preferably at most 3 ppm by weight. When theacetaldehyde content is within such a range, there will be no bad odoror abnormal smell derived from acetaldehyde, or there will be no changein the taste or flavor of the content, when the copolyester of thepresent invention is formed into a molded product such as a bottle or acontainer for food.

The above-described copolyester of the present invention can be preparedby melt polymerization, followed by solid-state polymerization inaccordance with a conventional method known with respect to PET.

Now, a process for its preparation will be described in detail. As amelt polymerization method, there is a method wherein an esterificationreaction is conducted directly under pressure using terephthalic acid,isophthalic acid and ethylene glycol and then the temperature is raisedand at the same time the pressure is gradually reduced to conduct apolycondensation reaction. Otherwise, an ester exchange reaction may beconducted using ester derivatives of terephthalic acid such as dimethylterephthalate and dimethyl isophthalate, and ethylene glycol, and thenthe obtained reaction product is further subjected to polycondensation.In these melt polymerization reactions, the isophthalic acid componentmay be added at any optional time i.e. during the esterificationreaction or the ester exchange reaction, or at the initial stage of thepolycondensation reaction. For example, an ester derivative ofterephthalic acid and ethylene glycol may first be subjected to an esterexchange reaction, and then isophthalic acid is added to the esterexchange reaction product, followed by polycondensation. Such apolycondensation reaction may be conducted in one step or in a dividedfashion in a plurality of steps. When the reaction is conducted in aplurality of steps, the polycondensation reaction conditions may be suchthat the reaction temperature for the first step of polycondensation isusually from 250° to 290° C., preferably from 260° to 280° C., and thepressure is usually from 500 to 20 mmHg, preferably from 200 to 30 mmHg,and the temperature of the final step of the polycondensation reactionis usually from 265° to 300° C., preferably from 270° to 295° C., andthe pressure is usually from 10 to 0.1 mmHg, preferably from 5 to 0.5mmHg.

When the polycondensation reaction is conducted in two steps, thepolycondensation reaction conditions for the first and second steps arein the above ranges, respectively. When the reaction is conducted inthree or more steps, the intermediate steps are conducted under thereaction conditions between the above-mentioned ranges.

For example, when the polycondensation reaction is conducted in threesteps, the reaction temperature for the polycondensation reaction of thesecond step is usually from 260° to 295° C., preferably from 270° to285° C., and the pressure is usually within a range of from 50 to 2mmHg, preferably from 40 to 5 mmHg.

There is no particular restriction as to the intrinsic viscosity reachedby each of these polycondensation reaction steps. However, it isdesirable that it is smoothly distributed in view of the degree ofincrease of the intrinsic viscosity in each step, and the intrinsicviscosity of the prepolymer obtained from the polycondensation reactorof the final stage is usually from 0.45 to 0.80 dl/g, preferably from0.50 to 0.70 dl/g, more preferably from 0.50 to 0.65 dl/g. If theintrinsic viscosity of the prepolymer is less than the above range,chipping tends to be difficult. On the other hand, if it exceeds theabove range, withdrawal of the prepolymer from the reactor tends to bedifficult, and the effects to reduce oligomers when the product issubjected to solid-state polymerization tend to be small. Usually, theprepolymer is withdrawn in the form of a strand from a molten state andthen cut into chips. Such chips usually preferably have an averageparticle diameter of from 2.0 to 5 mm, preferably from 2.2 to 4.0 mm.

In the esterification reaction, the ester exchange reaction and thepolycondensation reaction as described above, suitable amounts of anesterification catalyst, an ester exchange catalyst, a polycondensationcatalyst and a stabilizer are preferably employed.

The esterification catalyst may not necessarily be used, sinceterephthalic acid and isophthalic acid used serve as self-catalysts forthe esterification reaction. However, if necessary, a small amount of aninorganic acid may, for example, be used.

As an ester exchange catalyst, a known compound commonly used for PET,for example, at least one of calcium, titanium, manganese, zinc, sodiumand lithium compounds, may be employed. From the viewpoint oftransparency, a manganese compound is particularly preferred.

As a polycondensation catalyst, a known compound commonly used for PET,for example, at least one of germanium, antimony, titanium and cobaltcompounds may be employed. A germanium or antimony compound isparticularly preferred. In a case where transparency of the resultingcopolyester is important, it is preferred to employ a germaniumcompound. As the germanium or antimony compound, oxides, inorganic acidsalts, organic salts, halides and sulfides thereof may be mentioned.

In each of the ester exchange catalyst and the polycondensationcatalyst, such a catalyst is used usually in an amount within a range offrom 5 to 2000 ppm by weight, preferably from 10 to 500 ppm by weight,as the amount of metal in the total polymerization material. Especiallywhen a germanium compound is used, the amount of the compound is usuallywithin a range of from 10 to 100 ppm by weight, preferably from 30 to 60ppm by weight, more preferably from 35 to 55 ppm by weight, mostpreferably from 40 to 50 ppm by weight, as the content of germaniumatoms in the prepolymer or in the copolyester after the solid-phasepolymerization. Further, when an antimony compound is used, the amountis usually within a range of from 150 to 300 ppm by weight, preferablyfrom 170 to 280 ppm by weight, more preferably from 200 to 250 ppm byweight, as the content of antimony atoms in the prepolymer or in thecopolyester after the solid-state polymerization. When the content ofgermanium atoms or antimony atoms is within the above range, theoligomer-reducing rate and the solid state polymerization rate duringthe production of the copolyester by the solid-state polymerization ofthe prepolymer and the reduction in the formation of oligomers asbyproducts during the molding can be increased, such being advantageous.To satisfy the above ranges, for example, when germanium dioxide isused, from 50 to 300 ppm by weight, based on the polymer, of germaniumdioxide is usually employed, and when antimony trioxide is used, from180 to 1000 ppm by weight, based on the polymer, of antimony trioxide isusually employed at the time of the melt polymerization. However, thecontents may separately be controlled, for example, by adjusting thepolymerization temperature, pressure or time as well as the ratio of thedicarboxylic acid component to the glycol component of theesterification reactants.

As the stabilizer, an ester of phosphoric acid such as trimethylphosphate, triethyl phosphate, tri-n-butyl phosphate, trioctylphosphate, triphenyl phosphate or tricresyl phosphate, an ester ofphosphorous acid such as triphenyl phosphite, trisdodecyl phosphite ortrisnonylphenyl phosphite, an ester of acidic phosphoric acid such asmethyl acid phosphate, isopropyl acid phosphate, butyl acid phosphate,dibutyl phosphate, monobutyl phosphate or dioctyl phosphate, or aphosphorus compound such as phosphoric acid, phosphorous acid,hypophosphoric acid or polyphosphoric acid, is, for example, preferred.The stabilizer is used usually in an amount within a range of from 10 to1000 ppm by weight, preferably from 20 to 200 ppm by weight, as theweight of phosphorus atoms in the stabilizer in the total weight of thestarting material. Especially when a germanium compound is used as thepolycondensation catalyst, it is preferred to use the stabilizer so thatphosphorus atoms contained in the prepolymer or in the copolyester afterthe solid-state polymerization would be usually within a range of from0.3 to 1.5 times, preferably from 0.4 to 1.0 time, by the weight ratioto germanium atoms contained in combination. When the content ofphosphorus atoms is within this range, the thermal stability of theprepolymer or the copolyester obtainable by the solid-statepolymerization, will be good, and the oligomer-reducing rate during theproduction of the copolyester by the solid-state polymerization of theprepolymer, will be increased, such being advantageous.

Further, dicarboxylic acid component other than terephthalic acid andisophthalic acid, and a diol component other than ethylene glycol anddiethylene glycol, may be contained in small amounts, so long as theabove-mentioned requirements for the present invention are satisfied.Such a dicarboxylic acid component includes phthalic acid,2,6-naphthalenedicarboxylic acid, 4,4'-diphenylsulfonedicarboxylic acid,4,4'-biphenyldicarboxylic acid, 1,4-cyclohexanedicarboxylic acid,1,3-phenylenedioxydiacetic acid and structural isomers thereof, analiphatic dicarboxylic acid such as malonic acid, succinic acid oradipic acid, and an oxy acid or its derivative, such as p-hydroxybenzoicacid, a p-hydroxybenzoate, and glycolic acid. The diol componentincludes, for example, an aliphatic glycol such as 1,2-propanediol,1,3-propanediol, 1,4-butanediol, pentamethylene glycol, hexamethyleneglycol or neopentyl glycol, an alicyclic glycol such as cyclohexanedimethanol, and an aromatic dihydroxy compound derivative such asbisphenol A or bisphenol S. The composition (constituting units) of theprepolymer produced by the melt polymerization and the contents ofgermanium atoms, antimony atoms and phosphorus atoms are substantiallythe same as in the copolyester obtainable by subjecting the prepolymerto solid-state polymerization. However, more specifically, the contentsof such germanium atoms, antimony atoms and phosphorus atoms maysometimes decrease during the solid-state polymerization by an amount offrom 0 to 10%, respectively, relative to the contents of the respectiveatoms in the prepolymer.

It is advisable that AV of the prepolymer is usually within a range offrom 10 to 40 eq/ton, preferably from 15 to 30 eq/ton, more preferablyfrom 18 to 25 eq/ton. When AV is within such a range, the solid-statepolymerization rate during the solid-state polymerization of theprepolymer is high, and the oligomer-reducing effects are large, suchbeing advantageous. If AV is less than the above range, solid-statepolymerizability tends to be poor, whereby it tends to take a long timeto increase the intrinsic viscosity to a desired level. On the otherhand, if it exceeds the above range, the oligomer-reducing effects tendto be inadequate when the prepolymer is subjected to solid-statepolymerization.

Further, it is advisable that AV/TEV of the prepolymer is usually withina range of from 10 to 25 equivalent %, preferably from 12 to 22equivalent %, more preferably from 14 to 20 equivalent %. If theproportion of terminal carboxyl groups to the total terminal groups iswithin such a range, the solid-state polymerization rate and theoligomer-reducing rate during the solid-state polymerization of theprepolymer, are high, whereby the productivity will be improved, and thereduction in the formation of oligomers as byproducts during the moldingcan be furthered, such being advantageous.

The control of AV and AV/TEV of the prepolymer can be conducted by aconventional method for controlling AV and AV/TEV commonly employed inthe melt polymerization of PET, for example, by controlling the finalesterification ratio in the esterification reaction or the temperature,pressure or time during the ester exchange reaction or thepolycondensation reaction. The final esterification ratio in theesterification reaction can be controlled not only by the temperature,pressure or time of the esterification reaction but also by e.g. thefeeding ratio of the diol to the dicarboxylic acid or the refluxingratio (or the distillation rate) of formed water or diol. As a morespecific example, a case will be described wherein the prepolymer for acopolyester of the present invention is prepared by using a continuousmelt-polymerization installation commonly used for the preparation ofPET by direct polycondensation. In such a case, it is possible toincrease the final esterification ratio by prolonging the reaction timefor the esterification reaction, so that AV and AV/TEV of the prepolymerwill be lowered. However, more preferably, the final esterificationratio is increased by adjusting the refluxing ratio of ethylene glycolto a level larger than the case for ordinary PET, or by adjusting theesterification reaction temperature to a higher level, so that AV andAV/TEV of the prepolymer will be lowered. This is preferred from theviewpoint of the productivity of the prepolymer. Further, AV and AV/TEVof the prepolymer immediately prior to being subjected to solid-statepolymerization, may be controlled also by properly selecting themoisture-controlling conditions or the crystallization conditions forthe prepolymer, as mentioned in the foregoing, other than the controlduring the melt polymerization of the prepolymer as described above.

Then, in order to obtain a copolyester of the present invention, theprepolymer chips obtained by the melt polymerization as described above,are subjected to solid-state polymerization treatment.

The prepolymer chips to be subjected to solid-state polymerization maybe the ones having the moisture content adjusted by treating with water,steam or a steam-containing inert gas atmosphere, or may preliminarilybe heated at a temperature lower than the temperature for solid-statepolymerization, for preliminary crystallization and then supplied to thesolid-state polycondensation step. Such a preliminary crystallizationstep may be conducted by heating the prepolymer chips in a dried stateusually at a temperature of from 120° to 200° C., preferably from 130°to 180° C. for from one minute to 4 hours, or may be conducted byheating the chips in a steam atmosphere or a steam-containing inert gasatmosphere, usually at a temperature of from 120° to 200° C. for atleast one minute. Otherwise, such a preliminary crystallization step canbe conducted by permitting the prepolymer chips to absorb moisture in anatmosphere of water, steam or a steam-containing inert gas and thenheating the moisture-controlled prepolymer chips usually at atemperature of from 120° to 200° C. for at least one minute. Themoisture control of the prepolymer is carried out usually to such anextent that the water content of the prepolymer would be usually withina range of from 0.01 to 1% by weight, preferably from 0.1 to 0.5% byweight. By subjecting the water-containing prepolymer chips to acrystallization step or a solid-state polymerization step, it ispossible to further reduce the amount of acetaldehyde contained in thecopolyester of the present invention.

The solid-state polymerization process to which the above prepolymerchips are supplied, comprises at least one step, and the polymerizationtemperature is usually from 190° to 230° C., preferably from 195° to225° C. In a case of an inert gas stream method, the pressure is usuallywithin a range of from 1 kg/cm² G to 10 mmHg, preferably from 0.5 kg/cm²G to 100 mmHg, and the polymerization is conducted in an inert gasstream such as nitrogen, argon or carbon dioxide. In a case of a reducedpressure method, the pressure is usually from 0.01 to 300 mmHg,preferably from 0.01 to 100 mmHg. The solid-state polymerization time isusually from 1 to 50 hours, preferably from 5 to 30 hours, morepreferably from 10 to 25 hours, provided that the higher thetemperature, the shorter the time to reach the desired physicalproperties.

Bu properly selecting the conditions for the solid-state polymerizationtreatment as described above, a copolyester of the present invention canbe obtained.

The polyester of the present invention thus obtained may be formed intofilms, sheets, containers or other packaging materials, by means of amelt molding method which is commonly employed for PET. Further, themechanical strength can be improved by stretching the copolyester atleast in one axial direction.

To produce a stretched film, the stretching temperature may be set at alevel between the glass transition temperature of the copolyester of thepresent invention and a temperature higher by 70° C. than the glasstransition temperature. It is usually from 40° to 170° C., preferablyfrom 60° to 160° C. Stretching may be monoaxial or biaxial. However,biaxial stretching is preferred from the viewpoint of the practicalphysical properties of the film. The stretching ratio is usually withina range of from 1.1 to 10 times, preferably from 1.5 to 8 times, in thecase of monoaxial stretching, and within a range of from 1.1 to 8 times,preferably from 1.5 to 5 times in each of the longitudinal direction andthe transverse direction. Further, the ratio of the stretching ratio inthe longitudinal direction to the stretching ratio in the transversedirection is usually from 0.5 to 2, preferably from 0.7 to 1.3. Thestretched film thus obtained, is further subjected to heat setting toimprove the heat resistance and mechanical strength. The heat setting iscarried out usually under tension at a temperature of from 120° C. tothe melting point, preferably from 150° to 230° C., usually for from afew seconds to a few hours, preferably from a few tens seconds to a fewminutes.

To produce a hollow molded product, the preform prepared from thecopolyester of the present invention, is subjected to stretch blowmolding. For this purpose, a conventional apparatus commonly employedfor blow molding of PET, can be employed. Specifically, for example, apreform is first molded by injection molding or extrusion molding, andsubjected directly, or after processing the mouth portion and the bottomportion and reheating the processed preform, to a biaxially stretchingblow molding method such as a hot parison method or a cold parisonmethod. In such a case, the molding temperature, specifically thetemperature of various parts of the cylinder and the nozzle of themolding machine, can be set at a temperature usually lower by from 1° to10° C. than in the case for ordinary PET, usually within a range of from260° to 280° C., whereby the amount of oligomers can readily becontrolled to a low level. Further, lowering of the intrinsic viscositycan be controlled at a low level, and the amount of acetaldehydeproduced as a byproduct can readily be controlled at a low level. Thestretching temperature is usually from 70° to 120° C., preferably from80° to 110° C., and the stretching ratio may be usually within a rangeof from 1.5 to 3.5 times in the longitudinal direction and from 2 to 5times in the circumferential direction.

A hollow molded product thus obtained, may be used as it is. However,especially for the content which requires hot filling such as a fruitjuice beverage or Chinese tea, the hollow molded product is usuallysubjected to heat setting in the blow mold to impart heat resistancebefore use. The heat setting is conducted usually under tension by airpressure at a temperature of from 100° to 200° C., preferably from 120°to 180° C., for from a few seconds to a few hours, preferably from a fewseconds to a few minutes.

Now, the present invention will be described in further detail withreference to Examples. However, it should be understood that the presentinvention is by no means restricted to such specific Examples. In thefollowing Examples, "parts" means "parts by weight".

Various measuring methods employed in these Examples are as follows. Themeasuring methods for the intrinsic viscosity and the density were asdescribed above.

(1) Amount of isophthalic acid (hereinafter referred to simply as "IPAamount")

A sample was subjected to methanol-addition decomposition by a usualmethod, and the resulting dimethyl ester component was quantitativelyanalyzed by gas chromatography.

(2) Amount of diethylene glycol (hereinafter referred to simply as "DEGamount")

A sample was subjected to hydrolysis by a usual method, and theresulting diol component was quantitatively analyzed by gaschromatography.

(3) Content of a cyclic trimer (hereinafter referred to simply as "CTamount")

200 mg of a sample was dissolved in 2 ml of a liquid mixture ofchloroform/hexafluoroisopropanol (volume ratio: 3/2) and further dilutedby an addition of 20 ml of chloroform. To this solution, 10 ml ofmethanol was added to reprecipitate the sample, followed by filtrationto obtain a filtrate. The filtrate was evaporated to dryness, and theresidue was dissolved in 25 ml of dimethylformamide. With respect to thesolution thus obtained, the content of a cyclic trimer wasquantitatively analyzed by liquid chromatography.

(4) Concentration of terminal carboxyl groups (hereinafter referred tosimply as "AV")

100 mg of a copolyester sample was dissolved under heating in 5 ml ofbenzyl alcohol and diluted by an addition of 5 ml of chloroform. Then,the terminal carboxyl group concentration was quantitatively analyzed bytitration of a 0.1N sodium hydroxide/benzyl alcohol solution usingPhenol Red as an indicator.

(5) Concentration of the total terminal groups (hereinafter referred tosimply as "TEV")

Using the following equation, it was calculated from the intrinsicviscosity.

    TEV={2000000/(1359×intrinsic viscosity).sup.1.460 }

(unit: eq/ton)

(6) The ratio of terminal carboxyl groups to the total terminal groups(hereinafter referred to simply as "AV/TEV")

AV was divided by TEV, and the value thereby obtained was multiplied by100 to show the percentage (unit: equivalent %).

(7) Content of germanium atoms (hereinafter referred to simply as "Geamount")

2.0 g of a copolyester sample was ashed and completely decomposed by ausual method in the presence of sulfuric acid and then adjusted to avolume of 100 ml with distilled water, whereby the germanium content wasquantitatively analyzed by an emission spectral analysis.

(8) Content of antimony atoms (hereinafter referred to simply as "Sbamount")

In the same manner as the analysis of the germanium content, theantimony content was quantitatively analyzed by an emission spectralanalysis.

(9) Content of phosphorus atoms (hereinafter referred to simply as "Pamount")

In the same manner as the analysis of the germanium content, thephosphorus content was quantitatively analyzed by an emission spectralanalysis.

(10) Glass transition temperature (Tg) and low temperaturecrystallization temperature (Tc)

Measured by the method described in the foregoing using SEIKO I & E,SSC/580 (DSC20) Model Differential Scanning Calorimeter (manufactured bySeiko Denshi Kogyo K.K.).

(11) Content of acetaldehyde (hereinafter referred to simply as "AAamount")

A sample was extracted with water at 160° C. for 2 hours, whereupon theacetaldehyde content was quantitatively analyzed by gas chromatography.

(12) Solid-state polymerization rate

The increasing rate of the intrinsic viscosity per unit hour during thesolid-state polymerization, obtained by dividing the difference betweenthe intrinsic viscosity of the product obtained by the solid-statepolymerization and the intrinsic viscosity of the prepolymer by thesolid-state polymerization time, was taken as the solid-statepolymerization rate (unit: dl/g/hr).

(13) Inert gas flow rate

The inert gas flow rate was represented by the volume at 25° C. under 1atm of the gas passed through the unit weight of resin (kg) per unittime (hr).

EXAMPLE 1

A slurry comprising 12560 parts of terephthalic acid, 394 parts ofisophthalic acid and 5820 parts of ethylene glycol, was prepared andgradually supplied over a period of 4 hours to an esterification tankhaving 300 parts of bis(2-hydroxyethyl)terephthalate previously addedand maintained at 250° C. After completion of the supply, the mixturewas maintained at 250° C. for one hour for esterification. Then, onehalf was transferred to a polycondensation tank and 1.14 parts ofphosphoric acid (150 ppm by weight to the polymer) and 0.91 part ofgermanium dioxide (120 ppm by weight to the polymer) were charged. Thetemperature was gradually raised from 250° C. to 280° C., while thepressure was gradually reduced from an ordinary pressure and maintainedat a level of 0.5 mmHg. The reaction was conducted for 3 hours. Then,the formed prepolymer was withdrawn in the form of a strand from theoutlet provided at the bottom of the polycondensation tank, cooled bywater and then cut into chips. The analytical results of the prepolymerchips thereby obtained are shown in Table 1.

EXAMPLE 2

Prepolymer chips were prepared in the same manner as in Example 1 exceptthat 12700 parts of terephthalic acid and 263 parts of isophthalic acidwere used. The analytical results of the prepolymer chips are shown inTable 1.

EXAMPLE 3

Prepolymer chips were prepared in the same manner as in Example 2 exceptthat 197 parts of isophthalic acid and 1.10 parts of germanium dioxide(145 ppm by weight to the polymer) were used. The analytical results ofthe prepolymer chips are shown in Table 1.

EXAMPLE 4

14850 parts of dimethyl terephthalate, 150 parts of dimethylisophthalate, 10600 parts of ethylene glycol and 2.60 parts of manganeseacetate tetrahydrate (175 ppm to the polymer) were charged to a reactor,and the temperature was gradually raised from 160° C. to 220° C. over aperiod of 4 hours, and an ester exchange reaction was conducted whiledistilling off methanol formed by the reaction.

To this reaction product, 2.67 parts of phosphoric acid (180 ppm byweight to the polymer) and 1.78 parts of germanium dioxide (120 ppm byweight to the polymer) were added, and polymerization was conducted for3 hours at a final temperature of 275° C. under 0.5 mmHg to obtainprepolymer chips. The analytical results of the prepolymer chips areshown in Table 1.

EXAMPLE 5

Prepolymer chips were prepared in the same manner as in Example 2 exceptthat 1.90 parts of phosphoric acid (250 ppm by weight to the polymer)and 2.74 parts of antimony trioxide (360 ppm by weight to the polymer)instead of germanium dioxide were used, and the polymerization time waschanged to 2.5 hours. The analytical results of the prepolymer chips areshown in Table 1.

EXAMPLE 6

Prepolymer chips were prepared in the same manner as in Example 5 exceptthat 1.06 parts of phosphoric acid (140 ppm by weight to the polymer)and 2.36 parts of antimony trioxide (310 ppm by weight to the polymer)were used. The analytical results of the prepolymer chips are shown inTable 1.

EXAMPLE 7

Prepolymer chips were prepared in the same manner as in Example 5 exceptthat the esterification reaction time after completion of supplying thestarting material slurry was changed to one hour and 30 minutes. Theanalytical results of the prepolymer chips are shown in Table 1.

EXAMPLE 8

Prepolymer chips were prepared in the same manner as in Example 1 exceptthat the esterification reaction time after completion of supplying thestarting material slurry was changed to 40 minutes. The analyticalresults of the prepolymer chips are shown in Table 1.

EXAMPLE 9

An ester exchange reaction was conducted in the same manner as inExample 4 except that 14800 parts of dimethyl terephthalate, 225 partsof dimethyl isophthalate and 9600 parts of ethylene glycol were used. Tothis reaction product, 1.26 parts of cobalt acetate tetrahydrate (85 ppmby weight to the polymer) was added, and 1.80 parts of phosphoric acid(120 ppm by weight to the polymer) and 4.30 parts of antimony trioxide(290 ppm by weight to the polymer) instead of germanium dioxide wereused. Otherwise, the operation was conducted in the same manner as inExample 4 to obtain prepolymer chips. The analytical results of theprepolymer chips are shown in Table 1.

EXAMPLE 10

Prepolymer chips were prepared in the same manner as in Example 5 exceptthat 130 parts of isophthalic acid, 1.29 parts of phosphoric acid (170ppm by weight to the polymer) and 2.60 parts of antimony trioxide (345ppm by weight to the polymer) were used. The analytical results of theprepolymer chips are shown in Table 1.

COMPARATIVE EXAMPLE 1

Prepolymer chips were prepared in the same manner as in Example 2 exceptthat no isophthalic acid was added. The analytical results of theprepolymer chips are shown in Table 1.

COMPARATIVE EXAMPLE 2

Prepolymer chips were prepared in the same manner as in Example 5 exceptthat no isophthalic acid was added. The analytical results of theprepolymer chips are shown in Table 1.

COMPARATIVE EXAMPLE 3

Prepolymer chips were prepared in the same manner as in Example 2 exceptthat 12500 parts of terephthalic acid and 814 parts of isophthalic acidwere used. The analytical results of the prepolymer chips are shown inTable 1.

COMPARATIVE EXAMPLE 4

Prepolymer chips were prepared in the same manner as in Example 2 exceptthat 350 parts of diethylene glycol was added to the prepared slurry.The analytical results of the prepolymer chips are shown in Table 1.

COMPARATIVE EXAMPLE 5

Prepolymer chips were prepared in the same manner as in Example 2 exceptthat the esterification reaction time after completion of supplying thestarting material slurry was shortened to 30 minutes. The analyticalresults of the prepolymer chips are shown in Table 1.

COMPARATIVE EXAMPLE 6

Prepolymer chips were prepared in the same manner as in Example 5 exceptthat the esterification reaction time after completion of supplying thestarting material slurry was shortened to 30 minutes. The analyticalresults of the prepolymer chips are shown in Table 1.

COMPARATIVE EXAMPLE 7

Prepolymer chips were prepared in the same manner as in Example 2 exceptthat 0.45 part of germanium dioxide (65 ppm by weight to the polymer)was used. The analytical results of the prepolymer chips are shown inTable 1.

COMPARATIVE EXAMPLE 8

Prepolymer chips were prepared in the same manner as in Example 2 exceptthat 0.674 part of phosphoric acid (90 ppm by weight to the polymer) and1.37 parts of antimony trioxide (180 ppm by weight to the polymer)instead of germanium dioxide were used. The analytical results of theprepolymer chips are shown in Table 1.

EXAMPLE 11

The surface of the prepolymer chips obtained in Example 1 werecrystallized at 150° C. by a stirring-type crystallizing machine(manufactured by Bepex Co.). Then, the prepolymer chips were transferredto a solid-state polymerization tower, dried under a nitrogen stream of20 l/kg/hr at about 150° C. for 3 hours and then subjected tosolid-state polymerization at 208° C. for 20 hours to obtain solid-statepolymerization chips. The analytical results of the solid-statepolymerization chips are shown in Table 2.

EXAMPLES 12 to 23

Using prepolymer chips obtained in Examples 1 to 10 solid-statepolymerization was conducted for 20 hours at 208° C. or 215° C. in thesame manner as in Example 11 to obtain solid-state polymerization chips.The analytical results of the solid-state polymerization chips are shownin Table 2.

COMPARATIVE EXAMPLES 9 to 18

Using prepolymer chips obtained in Comparative Examples 1 to 8,solid-state polymerization was conducted for 20 or 30 hours at 208° C.in the same manner as in Example 11 to obtain solid-state polymerizationchips. The analytical results of the solid-state polymerization chipsare shown in Table 2.

EXAMPLE 24

Using solid-state polymerization chips obtained in Example 11, a preformwas molded by an injection molding machine IS-60B Model, manufactured byToshiba Corporation set for a temperature of various parts of thecylinder and the nozzle head of 270° C., a screw rotational speed of 100rpm, an injection time of 10 seconds and a mold quenching watertemperature of 10° C. This preform was, after heating and crystallizingthe mouth portion by a crystallizing machine, blow-molded by a stretchblow molding machine set for a preheating furnace temperature of 90° C.,a blow pressure of 20 kg/cm² and a molding cycle of 10 seconds to form abottle having a barrel portion wall thickness of 300 μm and having aninternal capacity of 1 l, followed by heat setting for 10 seconds underan air-pressure tension in the mold set at 150° C. The analyticalresults of the heat set bottle are shown in Table 3.

Further, as a continuous molding test, 1000 bottles were continuouslymolded, whereby no contamination of the mold was observed in each of theinjection, blow molding and heat setting operations.

Further, as a hot filling test, an orange fruit juice sterilized at 90°C. and left to cool to 85° C. was filled in the heat set bottle, andsealed with a cap. And the bottle was placed upside down for 15 minutes,whereupon a deformation of the bottle was inspected whereby no leakageof the juice or no deformation of the mouth portion, shoulder portionand the barrel portion was observed.

EXAMPLES 25 to 35

Using solid-state polymerization chips obtained in Examples 13 to 23,heat set bottles were molded in the same manner as in Example 24 exceptthat the temperature of various parts of the cylinder and the nozzlehead of the injection molding machine was changed to 265° C. or 275° C.The analytical results of the bottles are shown in Table 3.

Further, continuous molding tests were conducted in the same manner asin Example 24, and the molds after the molding were inspected, wherebyno contamination was observed in the mold of each of the injection, blowmolding and heat setting operations in each case.

Further, hot filling tests were conducted in the same manner as inExample 24, whereby no leakage or no deformation at the mouth portion,the shoulder portion and the barrel portion was observed in each of theheat set bottles prepared by using the respective solid-statepolymerization chips.

COMPARATIVE EXAMPLES 19 to 30

Using solid-state polymerization chips obtained in Comparative Examples9 to 18, heat set bottles were molded in the same manner as in Example24 except that the temperature of various parts of the cylinder and thenozzle head of the injection molding machine was changed to 270° C. or275° C. The analytical results of the bottles are shown in Table 3.

When injection molding was conducted at a temperature of various partsof the cylinder and the nozzle head of 270° C. using solid-statepolymerization chips of Comparative Example 10 and 12, the obtainedpreforms had blushing, and it was impossible to conduct the moldingsatisfactorily. As the cause, the solid-state polymerization chips had ahigh melting point since no isophthalic acid component wascopolymerized, and they could not completely be melted at the injectionmolding temperature of 270° C.

Further, continuous molding tests were conducted in the same manner asin Example 24, whereupon the molds after the molding were inspected. Inthe case where the solid-phase polymerization chips of ComparativeExample 13 were employed, no contamination was observed in the mold ofeach of the injection, blow molding and heat setting operations. In thecases where the solid-state polymerization chips other than those ofComparative Example 13 were used, a white thin film-like deposition wasobserved in each case, although there was a certain difference in thedegree of the deposition. From the visual inspection, it was observedthat the larger the content of the cyclic trimer in the molded product,the more remarkable the contamination of the mold.

Further, hot filling tests were conducted in the same manner as inExample 24. With the heat set bottles molded from the solid-statepolymerization chips obtained in Comparative Examples 13 and 14, theshoulder portions and the barrel portions underwent substantialdeformations, and leakage of the juice from the mouth portions wasobserved.

With the heat set bottles molded from the solid-state polymerizationchips other than Comparative Examples 13 and 14, no leakage or nodeformation at the mouth portion, the shoulder portion and the barrelportion, was observed.

EXAMPLE 36

Using solid-state polymerization chips obtained in Example 13, a sheethaving a thickness of 300 μm was molded by an extruder of 30 mm indiameter set for a temperature of various parts of the cylinder andvarious parts of the nozzle of 275° C., a screw rotational speed of 40rpm and an extruding amount of 80 g/min. The extrusion molding wascontinued for 10 hours, whereby no substantial contamination of thecooling dram was observed. Further, this extruded sheet wassimultaneously biaxially stretched at a stretching rate of 3×3 times bya Long biaxial stretching machine (manufactured by T. M. Long Co.) setfor the internal temperature of 90° C. and then heat-set in an oven at200° C. for 120 seconds under tension to obtain a stretched film havinga thickness of 30 μm. The analytical results of the films are shown inTable 4.

EXAMPLE 37 to 40

Using solid-state polymerization chips obtained in Examples 13, 16, 18and 22, sheets having a thickness of 300 μm were molded in the samemanner as in Example 36 except that the temperature of various parts ofthe cylinder and various parts of the nozzle was set at 270° C. or 275°C. The extrusion molding was continued for 10 hours, whereby transparentsheets were molded satisfactorily in each case, and no substantialcontamination of the cooling dram was observed. Further, using theseextrusion molded sheets, biaxially stretched heat set films having athickness of 30 μm were prepared in the same manner as in Example 36.The analytical results of the films are shown in Table 4.

COMPARATIVE EXAMPLES 31 and 32

Using solid-state polymerization chips obtained in Comparative Example9, a sheet having a thickness of 300 μm was molded in the same manner asin Example 36 setting the temperature of various parts of the cylinderand various parts of the nozzle to a level of 270° C. or 275° C.

When the temperature of various parts of the cylinder and various partsof the nozzle was set at 275° C., a transparent sheet was obtained.However, when the extrusion molding was continued for 10 hours,deposition of a white thin film was observed on the cooling dram.Further, using this extruded sheet, a biaxially stretched heat set filmhaving a thickness of 30 μm was prepared in the same manner as inExample 36. The analytical results of the film are shown in Table 4.

On the other hand, when the temperature of various parts of the cylinderand various parts of the nozzle was set at 270° C., a blushed sheet wasobtained, and it was impossible to conduct the molding satisfactorily.The cause may be such that the solid-state polymerization chips had ahigh melting point, since no isophthalic acid component wascopolymerized, and they could not completely be melted at the injectionmolding temperature of 270° C.

As shown in the foregoing Examples, the copolyester of the presentinvention has a low oligomer content and produces little oligomers asbyproducts during the molding. Further, as compared with homopolymerizedPET, it can be molded at a low temperature, whereby the possibility ofproducing oligomers as byproducts during the molding can further beminimized. Accordingly, when the copolyester of the present invention ismolded, contamination of the mold scarcely occurs, and it is notnecessary to frequently clean the molding apparatus for the productionof molded products, whereby the productivity of the molded products suchas bottles, films or sheets, can be improved. Besides, the copolyesterof the present invention is excellent in the heat resistance andmechanical strength and thus is suitable as a molding material forcontainers for fruit juice beverages which require heat resistance.

Further, the copolyester of the present invention has a higholigomer-reducing rate during the solid-state polymerization step in theprocess for its preparation, whereby the desired oligomer content can beattained in a short period of time as compared with conventionalhomopolymerized PET. In addition, the rate of increase of the intrinsicviscosity is high, the desired intrinsic viscosity can be attained in anequal or shorter period of time as compared with conventionalhomopolymerized PET, and thus the productivity is very high.

Thus, the copolyester of the present invention has a significantly highindustrial value.

                                      TABLE 1                                     __________________________________________________________________________            Composition                                                                   IPA  DEG  Intrinsic                                                           amount                                                                             amount                                                                             viscosity                                                                          TEV  AV   AV/TEV                                                                              Ge amount                                                                           Sb amount                                                                           P amount                                                                            CT amount                    (mol %)                                                                            (mol %)                                                                            (dl/g)                                                                             (eq/ton)                                                                           (eq/ton)                                                                           (eq. %)                                                                             (wt. ppm)                                                                           (wt. ppm)                                                                           (wt.                                                                                (wt.                 __________________________________________________________________________                                                             %)                   Example 1                                                                             3.0  2.0  0.56 124  24.3 19.6  41     0    22    0.92                 Example 2                                                                             2.0  2.0  0.56 124  23.8 19.2  43     0    23    0.94                 Example 3                                                                             1.5  1.7  0.57 121  20.2 16.7  53     0    27    0.96                 Example 4                                                                             1.0  2.2  0.58 118  16.1 13.6  48     0    30    0.97                 Example 5                                                                             2.0  2.0  0.56 124  24.0 19.4   0    240   76    0.93                 Example 6                                                                             2.0  2.0  0.55 128  18.2 14.2   0    213   43    0.93                 Example 7                                                                             2.0  2.0  0.56 124  12.8 10.3   0    238   75    0.93                 Example 8                                                                             2.0  2.0  0.58 118  26.5 22.5   0    240   75    0.93                 Example 9                                                                             1.5  1.7  0.55 128  27.5 21.5   0    191   37    0.95                 Example 10                                                                            1.0  2.3  0.56 124  21.1 17.0   0    230   51    0.95                 Comparative                                                                           0.0  2.0  0.56 124  24.1 19.4  43     0    24    1.01                 Example 1                                                                     Comparative                                                                           0.0  2.0  0.56 124  23.7 19.1   0    242   75    1.00                 Example 2                                                                     Comparative                                                                           6.0  2.4  0.58 118  24.0 20.3  43     0    24    0.88                 Example 3                                                                     Comparative                                                                           2.0  4.0  0.58 118  23.3 19.7  43     0    23    0.88                 Example 4                                                                     Comparative                                                                           2.0  2.0  0.56 124  32.9 26.5  44     0    23    0.94                 Example 5                                                                     Comparative                                                                           2.0  2.0  0.55 128  32.5 25.4   0    239   76    0.93                 Example 6                                                                     Comparative                                                                           2.0  2.0  0.56 124  23.3 18.8  25     0    20    0.94                 Example 7                                                                     Comparative                                                                           2.0  2.0  0.56 124  23.6 19.0   0    120   28    0.93                 Example 8                                                                     __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________           Solid-state                                                                   polymerization                                                                             Composition                                                      condition    IPA    DEG    Intrinsic                                          Prepolymer                                                                          Temp.                                                                             Time                                                                             amount amount viscosity                                                                          TEV  AV   AV/TEV                              used  (°C.)                                                                      (hr)                                                                             (mol %)                                                                              (mol %)                                                                              (dl/g)                                                                             (eq/ton)                                                                           (eq/ton)                                                                           (eq. %)                      __________________________________________________________________________    Example 11                                                                           Ex. 1 208 20 3.0    2.0    0.80 73.8 13.2 17.9                         Example 12                                                                           Ex. 1 215 20 3.0    2.0    0.87 65.3 10.3 15.8                         Example 13                                                                           Ex. 2 208 20 2.0    2.0    0.80 73.8 12.6 17.1                         Example 14                                                                           Ex. 2 215 20 2.0    2.0    0.87 65.3  9.7 14.9                         Example 15                                                                           Ex. 3 208 20 1.5    1.7    0.79 75.2 11.2 14.9                         Example 16                                                                           Ex. 4 208 20 1.0    2.2    0.74 82.7  7.4  8.9                         Example 17                                                                           Ex. 5 208 20 2.0    2.0    0.78 76.6 13.0 17.0                         Example 18                                                                           Ex. 5 215 20 2.0    2.0    0.83 69.9 10.2 14.6                         Example 19                                                                           Ex. 6 208 20 2.0    2.0    0.74 82.7  8.3 10.0                         Example 20                                                                           Ex. 7 208 20 2.0    2.0    0.70 91.6  6.6  7.2                         Example 21                                                                           Ex. 8 208 20 2.0    2.0    0.85 67.5 16.8 24.9                         Example 22                                                                           Ex. 9 208 20 1.5    1.7    0.77 78.0 13.9 17.8                         Example 23                                                                           Ex. 10                                                                              208 20 1.0    2.3    0.76 79.5  9.6 12.1                         Comparative                                                                          Comp. 208 20 0.0    2.0    0.78 76.6 12.8 16.7                         Example 9                                                                            Ex 1                                                                   Comparative                                                                          Comp. 208 30 0.0    2.0    0.85 67.5 10.0 14.8                         Example 10                                                                           Ex 1                                                                   Comparative                                                                          Comp. 208 20 0.0    2.0    0.76 79.5 13.1 16.5                         Example 11                                                                           Ex 2                                                                   Comparative                                                                          Comp. 208 30 0.0    2.0    0.83 69.9 11.1 15.9                         Example 12                                                                           Ex 2                                                                   Comparative                                                                          Comp. 208 20 6.0    2.4    0.83 69.9 10.8 15.5                         Example 13                                                                           Ex 3                                                                   Comparative                                                                          Comp. 208 20 2.0    4.0    0.83 69.9 11.1 15.9                         Example 14                                                                           Ex 4 -Comparative                                                                   Comp.                                                                             208                                                                              20     2.0    2.0  0.83 69.9 20.3 29.0                    Example 15                                                                           Ex 5                                                                   Comparative                                                                          Comp. 208 20 2.0    2.0    0.83 69.9 19.9 28.4                         Example 16                                                                           Ex 6                                                                   Comparative                                                                          Comp. 208 20 2.0    2.0    0.73 84.3 13.3 15.8                         Example 17                                                                           Ex 7                                                                   Comparative                                                                          Comp. 208 20 2.0    2.0    0.68 93.6 10.5 11.2                         Example 18                                                                           Ex 8                                                                   __________________________________________________________________________           Ge    Sb          CT              AA    Solid-state                           amount                                                                              amount                                                                              P amount                                                                            amount                                                                              Density   amount                                                                              polymerization                        (wt. ppm)                                                                           (wt. ppm)                                                                           (wt. ppm)                                                                           (wt. %)                                                                             (g/cm.sup.3)                                                                       Tg (°C.)                                                                    (wt. ppm)                                                                           rate (dl/g/hr)                 __________________________________________________________________________    Example 11                                                                           41    0     22    0.31  1.402                                                                              76.6 2.3   0.012                          Example 12                                                                           41    0     22    0.25  1.405                                                                              76.8 2.0   0.016                          Example 13                                                                           43    0     23    0.30  1.404                                                                              77.0 2.1   0.012                          Example 14                                                                           43    0     23    0.24  1.406                                                                              77.2 2.0   0.016                          Example 15                                                                           53    0     27    0.24  1.404                                                                              77.3 2.1   0.011                          Example 16                                                                           48    0     30    0.25  1.403                                                                              77.5 2.1    0.0080                        Example 17                                                                            0    240   76    0.28  1.404                                                                              77.1 2.1   0.011                          Example 18                                                                            0    240   76    0.23  --   --   --    0.014                          Example 19                                                                            0    213   43    0.23  --   --   --     0.0095                        Example 20                                                                            0    238   75    0.24  --   --   --     0.0070                        Example 21                                                                            0    240   75    0.36  --   --   --    0.014                          Example 22                                                                            0    191   37    0.34  --   --   --    0.011                          Example 23                                                                            0    230   51    0.25  1.403                                                                              77.4 2.2   0.010                          Comparative                                                                          43    0     24    0.44  1.403                                                                              77.7 2.1   0.011                          Example 9                                                                     Comparative                                                                          43    0     24    0.33  1.405                                                                              77.9 2.0    0.0097                        Example 10                                                                    Comparative                                                                           0    242   75    0.40  --   --   2.1   0.010                          Example 11                                                                    Comparative                                                                           0    242   75    0.30  --   --   2.0    0.0090                        Example 12                                                                    Comparative                                                                          43    0     24    0.27  1.400                                                                              72.5 2.2   0.013                          Example 13                                                                    Comparative                                                                          43    0     23    0.39  1.397                                                                              72.3 2.8   0.013                          Example 14                                                                    Comparative                                                                          44    0     23    0.50  --   --   --    0.014                          Example 15                                                                    Comparative                                                                           0    239   76    0.57  --   --   --    0.014                          Example 16                                                                    Comparative                                                                          25    0     20    0.60  --   --   --     0.0065                        Example 17                                                                    Comparative                                                                           0    120   28    0.51  --   --   --     0.0060                        Example 18                                                                    __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                                 Hollow molded bottles Effects                                                                               Hot                        Solid-state                                                                            Preform molding                                                                           Intrinsic                                                                          CT         AA    prevent filling                    polymerization                                                                         Temp                                                                              Appearance                                                                            viscosity                                                                          amount                                                                             Tg Tc amount                                                                              contamination                                                                         test                       polyester used                                                                         (°C.)                                                                      of preform                                                                            (dl/g)                                                                             (wt. %)                                                                            (°C.)                                                                     (°C.)                                                                     (wt. ppm)                                                                           of a                                                                                  results            __________________________________________________________________________    Example 24                                                                            Example 11                                                                             270 ∘                                                                         0.77 0.34 76.6                                                                             158                                                                               9.8  ∘                                                                         ∘      Example 25                                                                            Example 13                                                                             275 ∘                                                                         0.77 0.34 77.0                                                                             156                                                                              10.8  ∘                                                                         ∘      Example 26                                                                            Example 14                                                                             265 ∘                                                                         0.85 0.26 77.3                                                                             159                                                                               8.9  ∘                                                                         ∘      Example 27                                                                            Example 15                                                                             275 ∘                                                                         0.75 0.29 77.0                                                                             156                                                                              11.4  ∘                                                                         ∘      Example 28                                                                            Example 16                                                                             275 ∘                                                                         0.70 0.30 77.4                                                                             155                                                                              11.5  ∘                                                                         ∘      Example 29                                                                            Example 17                                                                             275 ∘                                                                         0.75 0.32 77.1                                                                             135                                                                              11.2  ∘                                                                         ∘      Example 30                                                                            Example 18                                                                             265 ∘                                                                         0.81 0.25 -- -- --    ∘                                                                         ∘                                                                 1                  Example 31                                                                            Example 19                                                                             275 ∘                                                                         0.71 0.27 -- -- --    ∘                                                                         ∘      Example 32                                                                            Example 20                                                                             275 ∘                                                                         0.65 0.30 -- -- --    ∘                                                                         ∘      Example 33                                                                            Example 21                                                                             275 ∘                                                                         0.75 0.41 -- -- --    ∘                                                                         ∘      Example 34                                                                            Example 22                                                                             275 ∘                                                                         0.74 0.38 -- -- --    ∘                                                                         ∘      Example 35                                                                            Example 23                                                                             275 ∘                                                                         0.73 0.29 77.5                                                                             132                                                                              11.0  ∘                                                                         ∘      Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.74 0.56 77.8                                                                             155                                                                              12.0  x       ∘      Example 19                                                                            Example 9                                                             Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.81 0.46 77.9                                                                             157                                                                              11.7  x       ∘      Example 20                                                                            Example 10                                                            Comparative                                                                           Comparative                                                                            270 x       --   --   -- -- --    --      --                 Example 21                                                                            Example 10   (blushing)                                               Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.72 0.51 -- -- 12.1  x       ∘      Example 22                                                                            Example 11                                                            Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.79 0.44 -- -- 11.5  x       ∘      Example 23                                                                            Example 12                                                            Comparative                                                                           Comparative                                                                            270 x       --   --   -- -- --    --      --                 Example 24                                                                            Example 12   (blushing)                                               Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.79 0.36 72.4                                                                             183                                                                              11.0  ∘                                                                         x                  Example 25                                                                            Example 13                                                            Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.78 0.47 72.4                                                                             181                                                                              11.5  x       x                  Example 26                                                                            Example 14                                                            Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.78 0.62 -- -- --    x       ∘      Example 27                                                                            Example 15                                                            Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.79 0.65 -- -- --    x       ∘      Example 28                                                                            Example 16                                                            Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.70 0.70 -- -- --    x       ∘      Example 29                                                                            Example 17                                                            Comparative                                                                           Comparative                                                                            275 ∘                                                                         0.65 0.59 -- -- --    x       ∘      Example 30                                                                            Example 18                                                            __________________________________________________________________________     Appearance of preform:                                                        ∘: Good,                                                          x: poor                                                                       Effects to prevent contamination of a mold:                                   ∘: Good,                                                          x: poor                                                                       Hot filling test results:                                                     ∘: Good,                                                          x: poor                                                                  

                                      TABLE 4                                     __________________________________________________________________________                              Biaxially                                                                     stretched sheet                                                                          Effects to                                      Solid-state                                                                           Sheet molding                                                                            Intrinsic                                                                          CT    prevent                                         polymerization                                                                        Temp                                                                              Appearance                                                                           viscosity                                                                          amount                                                                              contamination                                   polyester used                                                                        (°C.)                                                                      of sheet                                                                             (dl/g)                                                                             (wt. %)                                                                             of a mold                                __________________________________________________________________________    Example 36                                                                           Example 13                                                                            275 ∘                                                                        0.76 0.35  ∘                            Example 37                                                                           Example 13                                                                            270 ∘                                                                        0.77 0.33  ∘                            Example 38                                                                           Example 16                                                                            275 ∘                                                                        0.69 0.31  ∘                            Example 39                                                                           Example 18                                                                            275 ∘                                                                        0.74 0.26  ∘                            Example 40                                                                           Example 22                                                                            275 ∘                                                                        0.73 0.30  ∘                            Comparative                                                                          Comparative                                                                           275 ∘                                                                        0.73 0.58  x                                        Example 31                                                                           Example 9                                                              Comparative                                                                          Comparative                                                                           270 x      --   --    --                                       Example 32                                                                           Example 9   (blushing)                                                 __________________________________________________________________________     Appearance of preform:                                                        ∘: Good,                                                          x: poor                                                                       Effects to prevent contamination of a mold:                                   ∘: Good,                                                          x: poor                                                                  

What is claimed is:
 1. A copolyester which comprises, as maincomponents, terephthalic acid as a dicarboxylic acid component andethylene glycol as a diol component and which is characterized by:(1)isophthalic acid as a dicarboxylic acid component being from 0.5 to 3.0mol %, (2) diethylene glycol as a diol component being from 1.0 to 2.5mol %, (3) the intrinsic viscosity being from 0.60 to 1.50 dl/g, (4) theconcentration of terminal carboxyl groups being at most 18 eq/ton, and(5) the content of a cyclic trimer being at most 0.40% by weight.
 2. Thecopolyester according to claim 1, wherein the proportion of terminalcarboxyl groups to the total terminal groups is from 7 to 25 equivalent%.
 3. The copolyester according to claim 1, wherein the content ofgermanium atoms is from 30 to 60 ppm by weight.
 4. The copolyesteraccording to claim 1, wherein the content of antimony atoms is from 150to 300 ppm by weight.
 5. The copolyester according to claim 1, whereinthe content of a cyclic trimer is at most 0.35% by weight.
 6. Acopolyester which is produced by solid-state polymerization of aprepolymer comprising, as main components, terephthalic acid as adicarboxylic acid component and ethylene glycol as a diol component andcharacterized by:(1) isophthalic acid as a dicarboxylic acid componentbeing from 0.5 to 3.0 mol %, (2) diethylene glycol as a diol componentbeing from 1.0 to 2.5 mol %, (6) the intrinsic viscosity being from 0.50to 0.70 dl/g, and (7) the concentration of terminal carboxyl groupsbeing from 15 to 30 eq/ton, and which is characterized by (1)isophthalic acid as a dicarboxylic acid component being from 0.5 to 3.0mol %, (2) diethylene glycol as a diol component being from 1.0 to 2.5mol %, (3) the intrinsic viscosity being from 0.60 to 1.50 dl/g, (4) theconcentration of terminal carboxyl groups being at most 18 eq/ton, and(5) the content of a cyclic trimer being at most 0.40% by weight.
 7. Thecopolyester according to claim 6, which is produced by solid-statepolymerization of the prepolymer wherein the proportion of terminalcarboxyl groups to the total terminal groups is from 10 to 25 equivalent%.
 8. The copolyester according to claim 6, which is produced bysolid-state polymerization of the prepolymer wherein the content ofgermanium atoms is from 30 to 60 ppm by weight.
 9. The copolyesteraccording to claim 6, which is produced by solid-state polymerization ofthe prepolymer wherein the content of antimony atoms is from 150 to 300ppm by weight.
 10. The copolyester according to claim 6, which isobtained by conducting the solid-state polymerization at a temperatureof from 190° to 230° C. under a pressure of from 1 kg/cm² G to 0.01 mmHgfor from 1 to 50 hours.
 11. The copolyester according to claim 6, whichis obtained by solid-state polymerization of the prepolymer, which isconducted after adjusting the moisture content of the prepolymer to alevel of from 0.01 to 1.0% by weight.
 12. The copolyester according toclaim 1, wherein the copolyester consists essentially of terephthalicacid and isophthalic acid as dicarboxylic acid component and ethyleneglycol and diethylene glycol as diol component.
 13. The copolyesteraccording to claim 1, wherein the copolyester consists of terephthalicacid and isophthalic acid as dicarboxylic acid component and ethyleneglycol and diethylene glycol as diol component.